Re: Dual sim
I've been using dual sim phones for a few years (Chinese Frankenphones) and they work absolutely fine. Receiving stuff is seamless - you may or may not know which line it is coming in from depending on software, but you don't have to so anything to set that up. You decide which sim to use when you make a call. send a text, connect to data or whatever. I am sure that you can also do this automatically as well, but that might be dependant on software. Bottom line, I have never had any connection issues (in about 9 or 10 countries around the world) and have many options for cheaper connectivity while travelling.
I pick up local PAYG SIM cards wherever I happen to be (these are a great deal in developing countries, just buy them on the street rather than buying them in the airport) and I have had some cards active for years now. I do sometimes get funny looks when I sit on the 'plane and open up my travel walleet and choose through 6 or 7 SIM cards, but I think that is getting less noticeable now as well. My Tesco card I use about once a year when I am back in the UK, but I have had this for years now. Just spent three weeks in the US where my T-Mobile card was great for data ($10 per week for unlimited) although calls and texts were still a bit expensive on that plan.
My current 'phone has one standard and one micro-SIM slot. I had a bad experience with using an adapter so I went with this as the best option on my last purchase. All my current 'travelling' SIM cars are the full-size ones, but I am sure you can get smaller ones now so I will probably not bother with this on my next 'phone.
I have even seen three SIM phones advertised (on DX.com or AliExpress), but I think that may be going a bit too far. Sometimes even this seems like a gimmick, but on quite a few occasions in the 5 or 6 years, have a local and international number has been very useful to me persomally and i don't think I will go back.